Members of the Laboratory of Non-coding RNA and Genome Rearrangements, together with a team at the Institut Jacques Monod at Université Paris Cité and I2BC at Université Paris-Saclay (France), have identified a Paramecium homolog of Gametocyte specific factor 1 (Gtsf1) and have shown that it is essential for the selective degradation of small RNAs.

The study answers one of the crucial questions in the biology of small RNAs – how transposable elements (“TEs”) are distinguished from the rest of the genome during the establishment of small RNA populations, which is necessary to ensure the specificity of TE repression pathways.

In the ciliate Paramecium, 25-nt scnRNAs specific to TEs recruit Polycomb Repressive Complex 2 (PRC2) to TE loci and trigger their elimination from the genome during the formation of the new somatic nucleus. The authors showed that Gtsf1 is a PIWI-interacting protein that acts downstream of scnRNA biogenesis and is required for the selective degradation of a subpopulation of scnRNAs corresponding to non-TE sequences. In the absence of Gtsf1 cells accumulate repressive histone modifications but with no specific enrichment on TEs, which leads to a block of TE elimination.

The demonstration of a function for Gtsf1 in control of small RNA metabolism in Paramecium represents an important and unexpected advance at the intersection of several fields. Gtsf1 homologs have previously been implicated in PIWI-dependent TE repression, but the mechanism of action revealed in this work is completely different from those described in other organisms.

The results of the study were published in the prestigious journal Nucleic Acids Research. NAR reviewers and editors have given the work the status of a “Breakthrough Article”, which marks studies that solve a long-standing problem in their field or provide exceptional new insight and understanding into an area of research that will clearly motivate and guide new research opportunities and directions.

The work was supported by the National Science Centre of Poland (OPUS grants for JKN and SONATINA for JG).

The article is available at: https://doi.org/10.1093/nar/gkae1055